Question: Why can't white blood cells be injected to people who don't have enough like we do with red blood cells? Is it because they might start attacking the body because it dosen't recognise it?

Hi Jackie. Great question!
You’re actually right… White blood cells are your bodies main line of defense if a foreign invader has “breached” the outer “defensive wall” (the skin!). There are many different white blood cells, also called erythrocytes; and their function of recognizing “self” vs “foreign” is the reason behind graft/organ rejection. White and red blood cells are produced inside the bone, by specific stem ells that sit inside bone marrow.
If someone has low levels of white blood cells, they are at risk of infections and diseases. The only fix is to replenish/ change the stem cells that produce white blood cells, often a bone marrow transplant, so that the correct amount of white blood cells are produced. This means someone (usually from your family) who’s bone marrow is an identical match to your bone marrow, can donate bone marrow to you.

Hope this makes sense 😀